The present invention relates to casing structure of a transmitter of a tire condition monitoring apparatus. More particularly, the present invention pertains to casing structure of a transmitter for a wireless tire condition monitoring apparatus that permits a driver in a vehicle passenger compartment to check conditions of tires, such as the air pressure.
Wireless tire condition monitoring apparatuses that allow a driver in a vehicle passenger compartment to check the conditions of vehicle tires have been proposed. The apparatus of the publication includes transmitters and a receiver. Each transmitter is located in the wheel of one of the tires and the receiver is located in the body frame of the vehicle. Each transmitter detects the conditions, such as air pressure and the temperature of the associated tire, and wirelessly transmits the detected information. The receiver receives data wirelessly transmitted by the transmitters with an antenna and displays the conditions of the tires, for example, on a display located in front of the driver's seat.
Each transmitter is formed by electronic elements such as a pressure sensor and a transmission antenna, which are mounted on a substrate. The substrate on which the electronic elements are mounted is accommodated in a casing. The casing is filled with, for example, silicon potting material except around a diaphragm of the pressure sensor to maintain the environment resistance such as the moisture resistance and the water resistance of the transmitter. As a result, a first surface (the upper portion) and an opposite second surface (the lower portion) of the substrate are covered with potting material except around the diaphragm of the pressure sensor. A lid that covers the opening of the casing has a vent hole for permitting the pressure sensor to measure the air pressure in the associated tire. Therefore, the air pressure in each tire and the air pressure in the corresponding casing are maintained to be the same (see Japanese Laid-Open Patent Publication No. 2003-112506).
However, the second surface of the substrate is covered with potting material. The potting material is covered with incompressible casing. That is, the space between the second surface of the substrate and the casing is filled with potting material. As a result, when the tire is inflated, the air pressure is applied to the incompressible casing and is hardly applied to the potting material on the second surface of the substrate. On the other hand, the first surface of the substrate is covered with potting material except around the diaphragm of the pressure sensor. As a result, when the tire is inflated, the air pressure is applied to the potting material on the substrate via the vent hole.
Since the potting material is compressible, the potting material is elastically deformed when receiving the air pressure. That is, the air pressure is reduced by the compressible potting material. In other words, the air pressure is reduced by the potting material.
The periphery of the substrate is secured by a boss integrally formed with the casing. Therefore, the air pressure applied to the substrate via the potting material differs from the air pressure applied to the substrate via the pressure sensor, which causes the center of the substrate to flex downward. As a result, stress is applied to the electronic elements located at the flexed portion of the substrate. Thus, the substrate may be formed thicker taking into consideration of the air pressure in the tire. However, when the substrate is formed thicker, the weight of the transmitter is slightly increased. Although the weight increase of the transmitter is slight, the centrifugal force caused by the rotation of the tire is increased in proportion to the weight. Therefore, increasing the thickness of the substrate is undesirable. Furthermore, increasing the thickness of the substrate conflicts with the purpose of reducing the size of the casing, which accommodates the transmitter.